Code updates

[TomDonoghue]

Updates / additions to make to code:

• [ ] plots: for visualizing model components (plot_peak_fits and plot_aperiodic_fits), it would be nice to support the use case of plotting the mean + shading (sem or std). This is perhaps better for comparing groups. Note that this might need to be a new func.
• [ ] add average_model_values helper function. We currently average parameters (in average_fg), but a useful alternative would be to average the model reconstructions